The invention relates to a method of providing a substrate of an optically readable information disc with an optically detectable structure, which disc can be rotated by means of a drive apparatus comprising a drive spindle and optical read means. The mold has a base provided with a mold structure which is covered with a reproduction layer, which in a deformable phase adapts itself to the mold structure is subsequently solidified and, while attached to the substrate, is separated from the mold in such way that the structure is maintained.
U.S. Pat. No. 4,312,823 describes the manufacture of a video long-play disc by means of a reproduction layer made of a moulding resin. The substrate is transparent and is made of a suitable synthetic material. The moulding resin may be a radiation-curable moulding resin, which is exposed through the transparent substrate. The method described therein may also be employed for the manufacture of information discs for computer memories and similar uses, on which information can be recorded by means of a suitable light source, such as a laser.
U.S. Pat. No. 4,363,116 describes providing an information disc with an optically detectable servo track which extends over the entire area of that part of the information disc which is available for recording. The servo track is generally a spiral track, but in principle it may alternatively comprise a multitude of concentric sub-tracks. By means of a servo track it is possible to control the radial position of the radiation spot formed on the information layer by the radiation beam. The servo track may already contain information before the user records information on the information disc. This pre-recorded information in the servo track may comprise, for example, sector addresses in each of which the address of the associated continuous portion of the servo track is encoded in address areas. The servo track contains a multitude of sector addresses per turn of the servo track. In addition to the track number or the number of the turn of the spiral servo track, the sector addresses may, for example, also contain the number of the relevant sector in this track or in this turn. In addition, a plurality of synchronizing areas may be present at the beginning of each sector address, which when read yield a clock regeneration signal which dictates the clock frequency of the electronic clock which determines the frequency with which the information to be recorded is supplied. The layer on which recording is possible by optical means is deposited on the transparent layer provided with the servo track and may be a thin metal layer, for example one containing tellurium.
Generally, optically readable information discs should comply with very stringent requirements as regards the eccentricity of the structure, for example the servo track, relative to the axis of rotation of the information disc. In a known information disc on which recording can be made by optical means, the information disc comprises two glass substrates which are hermetically affixed to each other by means of interposed annular spacers, as in U.S. Pat. No. 4,074,282. Such a disc, which has a diameter of approximatrly 30 cm, contains 32,000 spiral grooves with a pitch of 1.6 micron. The eccentricity of the servo track relative to the axis of rotation of the information disc should not exceed 20 microns. It will be evident that when the servo track and the sector addresses are formed on the substrate the method of forming the structure on the substrate should comply with very stringent requirements in view of the extremely small permissible eccentricity.
The method described in U.S. Pat. No. 4,670,077 solves this problem and is suitable for forming a structure on a substrate of an information disc with a very small eccentricity, smaller than the required 20 microns. That method includes the following steps:
providing a central recess with a wall of circular cross section at the center of the base, PA0 arranging the mold structure on the base concentrically with the central recess, PA0 providing a disc hub with a central through-hole with a wall of circular cross-section intended for centering the finished information disc on the drive spindle of a drive apparatus, PA0 permanently attaching the hub to the center of the substrate to form an intermediate product, PA0 placing the intermediate product on the mold with the reproduction layer in its deformable phase therebetween, PA0 entering the intermediate product by means of auxiliary centering means which is inserted into the through hole of the hub and makes contact both with the wall of the central recess in the base and the wall of the through hole in the hub, PA0 fixing the intermediate product on the mold until the reproduction layer has solidified and adheres to the substrate. PA0 providing an auxiliary tool having a central recess with a wall of circular cross section; PA0 detachably connecting a structure carrier provided with a mold structure to an auxiliary substrate identical to the substrate used for the disc; PA0 centering the mold structure relative to the central of the auxiliary tool; PA0 centering a disc hub relative to the mold structure by using centering means which is inserted into the through hole in the hub and into the central recess of the auxiliary tool; PA0 fixing the hub thus centered to the auxiliary substrate; PA0 centering the assembly comprising the structure carrier, the auxiliary substrate and the hub on the base by using centering means which is inserted into the through hole of the hub and into the central recess of the base; PA0 fixing the structure carrier to the base; and PA0 detaching the auxiliary substrate from the mold structure.
The small eccentricity of the structure on the substrate obtained by means of this method is due to the fact that the accuracy with which the mold structure is centered relative to the base is transferred to the intermediate product through the cooperation between the disc-centring means and the base-centring means on th base of the mold. During use the finished information disc cooperates with the drive spindle of a drive apparatus using the same disc-centring means that were employed during manufacture for centring the information disc on the mould. Thus, the high accuracy of the centring on the mould is not lost at a later stage, which would happen if the disc-centring means would have been mounted at a later stage. A substantial advantage of the invention is that the method can be carried out with a short cycle time. This is because the method does not require any measurements and/or alignments. The optical information disc is manufactured by means of expensive equipment in clean rooms so that a very short cycle time is very desirable in order to reduce the cost price of the product.
Usually the mould structure is formed on a carrier obtained by an electroplating method and is secured to the base as a thin metal disc, generally referred to a "shell".
Although the information structure is intended for transparent glass or plastics substrates and with a reproduction layer made of a transparent rediation-curable moulding resin, non-transparent substrates and reproduction layers of a different type may be used. This layer may be a plastic foil which adapts itself to the mould structure under the influence of heat. It is also conceivable that a reproduction layer is formed by softening the surface of the substrate itself.
The base centering means may be a central recess in the base of the mould and the disc-centring means may be a central through-hole in the disc hub, the intermediate product is centered by means of an auxiliary-centering means which is inserted into the through-hole and makes contact both with the wall of the central recess in the base and with the wall of the through-hole in the hub.
The base-centering means may thus have a very simple shape and can be manufactured easily. Damaging of the base-centering means is practically impossible, because these means do not project from the base. The auxiliary centering means have a simple shape and yet contribute to a high centring accuracy. Preferably, the construction is such that the auxiliary centring means cooperate with the two said walls without clearance.
The disc hub may be axially deformable to a limited extend and when the auxiliary centring means are inserted they first abut against the wall of the through-hole in the disc hub and, subsequently, after an axial displacement, also abut against the wall of the central recess in the base of the mould. Since the auxiliary centring means abut both against the wall of the through-hole in the hub and against the wall of the central recess in the base of the mould, this ensures that for auxiliary centring means cooperate with the two other centring means without any clearance. As a result of the axial deformability of the hub the cooperation with these two centring means is independent of any tolerances in the axial distance between the base-centring means and the disc-centring means of an intermediate product which is mounted on the mould. These variations can only give rise to variations in the axial deformation of the hub relative to the mould.
A slight axial deformation of the hub is a centring principle which is also suitable for centring a finished information disc on a drive apparatus. U.S. Pat. No. 4,224,648 describes such a centring principle for a magnetic information disc. Since an axial deformation of the hub may lead to small displacements in a radial direction of the substrate relative to the mold, the axial deformation of the disc hub on the mould should have a predetermined value equal to the axial deformation to which the hub is subjected when the finished information is mounted on the spindle of a drive apparatus. Centering errors as a result of axial deformation of the hub when the finished information disc is mounted on a drive spindle have at most a very small influence on the eccentricity. This is because allowance is made for such errors when the substrate is provided with the structure.
The auxiliary centering means may be a spherical surface which engages with walls of the disc hub and the mold along circular lines of contact. This is especially suitable when the wall of the central recess in the base of the mould is situated on a conical surface and the wall of the through-hole in the disc hub is situated on a spherical surface of a diameter substantially equal to the spherical surface of the auxiliary centring means.
In order to enable the auxiliary centering means to be handled more conveniently, they may comprise a ball mounted on a stem. Balls having radii with very small tolerances are commerically available. A similar ball may be used on the spindle of the drive apparatus for which the information disc is intended.